1. Field of the Invention
This invention relates to the assembly of systems using semiconductor devices. Specifically, this invention relates to maintaining thermal efficiency between a microchip and a heat sink or cooling plate used to transport thermal energy away from the microchip. More specifically, the invention is directed to a flexible mounting plate that holds the microchip against a heat sink or cold plate while allowing the tandem to have z-direction movement without degrading the thermal bond between them.
2. Description of Related Art
In high performance computers, removal of heat from microchips is very important and critical to the proper operation of the microchips and ultimately to the successful operation of the system. Efficient heat removal is achieved by maintaining intimate thermal contact between a cooling plate or heat sink and a microchip during system assembly and throughout the product's lifecycle.
However, difficulties arise in maintaining constant contact between a heat sink or cooling plate and the microchips on a semiconductor substrate throughout the assembly process and the subsequent operation of the system during its lifetime. Importantly, the specifications for the cooling solutions require that thermal contact be maintained throughout the product life cycle, which may include ensuring contact under shock and vibration amplitudes on the order of 50 g. FIG. 1 illustrates a microchip/heat sink bond design of the prior art using spring mounting screws 32 for attaching a heat sink plate 34 to the printed circuit board 38, and pressing the pedestal 12 of the heat sink 30 against the microchip 36. The microchip is mounted on a semiconductor substrate 40 that is electrically attached to the printed circuit board 38 via solder balls 42.
A detriment of the prior art is that the mechanical structure of FIG. 1 creates a statically indeterminate situation where contact pressure between the microchip and a heat sink or cold plate becomes unknown, and is subject to excessive variability. Moreover, the design is prone to friction, stiction, and mechanical binding between screws 32 and clearance holes 35. The prior art design's variability is in all directions. Flexibility remains important to the mounting design, ensuring constant contact while under vibration and shock induced forces. The current prior art designs do not account or allow for this flexibility, and rely instead on a rigid configuration to ensure contact. In providing a rigid support, the prior art yields to separation forces that tend to move the microchip relative to the cooling device in x-, y-, and z-directions. Moreover, the current designs are prone to mounting screw binding due to the nature of the screws 32 and clearance holes 35. This binding or friction alters the interface pressure from optimum design criteria and creates a statically indeterminate mechanical design. The friction may also change over time and cause interface pressure changes.
Thus, for mechanical strength and reliability, the interface pressure between the microchip and the cooling plate or heat sink must be closely controlled. A typical required pressure is approximately in the range of 25 to 250 psi for a microchip mounted to a semiconductor substrate and the substrate mounted to a circuit board using solder balls. Thermal paste or grease 29 is used at the heat sink/microchip interface to maximize the heat flow. Once the heat sink, microchip, and thermal paste are forced together at 25 to 250 psi, the excess paste will extend outwards from the joining faces. In this position, the contacting surfaces should not be disturbed through rocking, twisting, sliding, or other separation forces. If the interface is disturbed, thermal performance is degraded, which may result in microchip overheating.
One solution has been to enlarge the holes 35 in the cooling plate or heat sink mounting plate 34 to eliminate potential binding and friction. Enlarged holes are preferred over straight bushings since they are less likely to cause binding when subjected to non-centered forces. They can also alleviate binding caused by slightly tilted posts. However, by doing this, the ability to accurately locate the heat sink 30 in the x-, y-, and theta-directions is compromised. Location accuracy in this plane is needed to prevent any sliding movement of the cooling device during thermal cycling, shock, and vibration environments.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus to secure a microchip to a cold plate or heat sink that will allow for component fitness variations and tilt, while ensuring intimate cold plate or heat sink contact to the microchip, and providing constraints in alignment in the x-, y-, and theta-directions.
It is another object of the present invention to provide an apparatus for attaching a microchip to a cooling device that does not contribute to friction, stiction, or binding during operation.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.